Does the iGP have a national office and where is it situated?

Yes, the India Greens Party has its national head office (NHO) based at — GreenDham AnandiChait, IndraBalbhadra Parisar, Unchir-Dunktok on Devprayag-Buwakhal State Highway-31, PO-Ghurdauri, Patty-Idwalsyun, Pauri Garhwal, Uttarakhand, PIN-246194. The Google location of the iGP.NHO is as follows:

https://www.google.com/maps/place/India+Greens+Party/@30.1777552,78.7034568,17z/data=!4m5!3m4!1s0x0:0xb122ede0ed8faeb7!8m2!3d30.1777552!4d78.7056455